This invention relates to an apparatus for electrophoresis. More particularly, this invention relates to a compact vertical-type apparatus for gel electrophoresis.
Gel electrophoresis is practiced extensively as a technique for determining the base sequences of DNA and other proteins. Conventionally, the sample to be subjected to electrophoresis is labelled with a radioisotope for analysis but this method has had the problem of being painstaking and time-consuming. Furthermore, the use of radioactive substances always calls for utmost safety and management and analysis cannot be performed in areas other than facilities that clear certain regulations. Under the circumstances, a method that uses fluorophores to label the sample and which detects fluorescences as emitted upon irradiation with light is being reviewed.
In this method, fluorophore-labelled DNA fragments are caused to migrate through a gel and a light excitation portion and a photodetector are provided for each electrophoresis track in an area 15-20 cm below the start point of electrophoresis. The DNA fragments are assayed as they pass through the line connecting the light excitation portion and the photodetector. A typical procedure of the method is described below. First, using as template the DNA chain to be determined for its base sequence, DNAs of various lengths with known terminal base species are replicated by a method involving an enzymatic reaction (the dideoxy method). Then, the replicated DNAs are labelled with a fluorophore. Stated more specifically, there are prepared a group of adenine (A) fragments, a group of cytosine (C) fragments, a group of guanine (G) fragments and a group of thymine (T) fragments, all being labelled with a fluorophore. A mixture of these fragment groups is injected into separate lane grooves in an electrophoretic gel and, thereafter, a voltage is applied at opposite ends of the gel. Since DNA is a chained polymer with negative charges, it will move across the gel at a rate in inverse proportion to its molecular weight. The shorter the DNA chain (the smaller its molecular weight), the faster will it move and vice versa; this is the principle behind the fractionation of DNA by molecular weight.
Japanese Laid-Open Patent Application (kokai) No. 21556/1988 teaches a DNA base sequencer that is adapted in such a way that a line on the gel in an apparatus for electrophoresis at which laser light is applied and the direction in which photodiodes are arranged are both perpendicular to the direction in which DNA fragments migrate in the apparatus. The setup of this apparatus is shown schematically in FIG. 6. An electrophoresis plate 74 comprises a gel (typically a polyacrylamide gel) held between two glass plates. The electrophoresis plate has an overall thickness of up to about 10 mm but the thickness of the gel electrolyte layer itself is less than about 1 mm. The upper end of the gel electrolyte layer is comb-shaped and located slightly below the upper end of the plate 74. Fluorophore-labelled DNA fragments to be assayed are injected into grooves 75 corresponding to the teeth of the comb.
In the apparatus shown in FIG. 6, a laser beam emitted from a light source 70 is reflected by a mirror 72 and launched horizontally from one side of the plate 74 at a predetermined point on the gel. As the fluorophore-labelled DNA fragments migrating through the gel pass through the irradiated region, they will fluoresce successively. The horizontal position of fluorescence emission tells the species of a particular terminal base, the time difference from the start of migration tells the length of a particular fragment, and the emission wavelength identifies the sample under assay. The fluorescence from each electrophoresis track is condensed by a lens 78 to focus at a light-receiving area in an image intensifier 80. The received signal is amplified and converted to an electric signal in a photodiode array 84 for the purpose of various measurements. The results of measurements are processed with a computer so that the sequences of the individual DNA fragments are calculated to determine the base sequence of the DNA at issue.
The conventional apparatus for gel electrophoresis uses an argon laser as a light source and adopts an image intensifier camera in the light-receiving optics. The image intensifier camera is not only very expensive but also comparatively large as an optical device. The argon laser is also of a comparatively large size and requires a large transformer for use with the associated drive power supply. As a result, the overall size of the conventional apparatus for electrophoresis becomes bulky, often making it difficult to allow for a space for accommodating signal processing boards and other related devices.